1. Field of the Invention
The present invention relates to a communication system such as an asynchronous transfer mode (ATM) system, and more particularly, to a cell rate supervising system used in the communication sytem.
2. Description of the Related Art
Recently, ATM communication systems have been developed to realize a broadband integrated services digital network (B-ISDN).
In the ATM communication system, all multi-media information related to audio, video and data are stored in fixed length cells each formed by a header (5 bytes) and a payload (48 bytes). Since the cells have the same configuration, it is possible to multiplex, demultiplex and switch in the same network at different rates regardless of the kind of multi-media.
In the ATM communication system, if an abnormally large number of cells are supplied to a network, it is impossible to guarantee quality of service (QOS). This state is called a congested state. For example, assume that a connection is carried out between a first source terminal and a destination terminal via the network and a connection is carried out between a second source terminal and the destination terminal via the network. In this case, when a cell rate from the first source terminal to the destination terminal plus a cell rate from the second source terminal to the destination terminal exceeds a peak cell rate (PCR) from the network to the destination terminal, a congested state may occur in the network.
In order to suppress the generation of congested states, various ATM service classes, i.e., a constant bit rate (CBR) service, a variable bit rate (VBR) service, an unspecified bit rate (UBR) service and an available bit rate (ABR) service are defined in the ATM communication system.
In the CBR service, a fixed cell rate is allocated to each connection between a terminal and the network. Therefore, even if there are a plurality connections carried out for one destination terminal via the network, a total of such fixed cell rates of the connections are caused to be lower than a PCR from the network to the destination terminal, and therefore, a congested state may not be generated.
In the VBR service, a statistically-determined variable cell rate is allocated to each connection between a terminal and the network. Even in this case, even if there are a plurality connections carried out for one destination terminal via the network, a total of such variable cell rates of the connections are caused to be lower than a PCR from the network to the destination terminal, and therefore, a congested state may not be generated.
In the UBR service, a cell rate is determined by a terminal, that is, the control of cell rates by the network is not carried out. Therefore, a congested state may be generated.
In the ABR service, an allowed cell rate (ACR) is calculated in accordance with congestion information fed back from the network, and also, the ACR is changed between a minimum cell rate (MCR) and a PCR. That is,
MCR.ltoreq.ACR.ltoreq.PCR
Thus, a feedback operation using the congestion information is performed upon the ACR, which effectively makes use of the network.
The present invention is related to the ABR service.
Even in the ABR service, a congested state may be generated. That is, if a terminal has a trouble, the terminal may generate cells beyond the ACR. Or, if the MCR of the terminal is erroneously changed, the terminal may generate cells beyond the optimum ACR. Therefore, in order to monitor whether or not the cell rate of cells generated from each terminal is lower than the corresponding ACR, a cell rate supervising unit having a policing function is provided. In the ABR service, a dynamic generic cell rate algorithm (DGCRA) is used.
Note that a generic cell rate algorithm (GCRA), a virtual scheduling algorithm (VSA) or a continous-state leaky bucket algorithm (CSLBA) are used for the CBR service and the VBR service. For example, a usage parameter control (UPC) unit using these algorithms monitors excess traffic in a user-network interface (UNI), and a network parameter control (NPC) unit using these algorithms monitors excess traffic between networks in a network-node interface (NNI).
Returning to the DGCRA in the ABR service, an ACR is calculated in the cell rate supervising unit in the same way as in the terminal. Therefore, if a congestion information cell is erroneously scrapped to interrupt the feedback operation, the ACR calculated in the terminal does not coincide with the ACR calculated in the cell rate supervising unit. Therefore, the policing operation cannot be carried out normally. As a result, accessible cells may be scrapped or transmitted with special tags, or unaccessible cells may be transmitted through the cell rate supervising unit. The former makes a user terminal disadvantageous. The latter not only makes the user terminal disadvantageous, but also reduces the link utilization of the network. Although this abnormal state may soon disappear in an explicit rate (ER) mode, this abnormal state may not disappear for long time in a binary mode. This will be explained later in detail.